1. Field of the Invention
This invention relates to compatible polymer blends useful as melt adhesives comprising (1) a low density ethylene polymer, (2) a copolymer of butene-1 and ethylene or propylene, (3) a hydrocarbon tackifier and (4) a low molecular weight polymer selected from the group consisting of (a) a low molecular weight liquid polybutene, (b) an amorphous (atactic) polypropylene and (c) mixtures thereof.
2. Description of the Prior Art
Hot melt adhesives have found extensive use in industry in bonding, joining or fabrication of various structures, including construction of structures from synthetic polymeric films, such as polyethylene, polypropylene, etc.; foil, including metal foil, such as aluminum; wax-coated or wax-impregnated cellulosic structures; and various non woven materials whose constituents are based on polyolefins, polyesters, polyamides and acrylic-type polymers.
The modern trend in the packaging industry, for example, is to use more of the plastic based or plastic finished constructions, such as polyethylene or polypropylene extrusion coated or laminated boards. Such structures are replacing conventional clay coated boards in many packaging applications. The advantages of using composite construction of plastic to plastic or plastic to paper, and other such combinations, are several, for example, with respect to aesthetics, print graphics, barrier properties toward moisture (humidity), oxygen and order, etc. A better barrier, for example, will prevent or tend to inhibit early spoilage of the packaged contents. Plastic utilized composite constructions are now routinely used for beverage cartons or sleeves, to hold dry goods and consumable items, frozen juice, household detergents and cleaners, poultry and fresh produce packaging, etc.
The change in various packaging laminate and non woven disposal construction structures from paper or paperboard to plastic and non-woven based products also necessitates that the various adhesives chosen to hold the structures together be compatible with the materials of construction, since the altered surface chemistry of such materials may not be adhesion compatible with traditionally available hot melt adhesives.
Thus, it is important that the hot melt adhesives used to prepare the various composites be compatible with the materials of construction and that they exhibit good specific adhesion thereto, particularly wherein such materials are made using synthetic polymers, such as polyethylene, polypropylene, etc.
In addition, since hot melt adhesives are a multicomponent system which experiences prolonged elevated temperature exposures, for example at 350.degree. F., or even higher, in production or use, it is especially important that they show melt stability over a period of time unitl their use and not lose their homogeneity, become unstable and show stratification, resulting in a substantial loss in their adhesive performance. Moreover, some hot melt adhesives, even at such elevated temperatures, are still so viscous, for example, they can have a viscosity at such a temperature well above 15,000 cps, that they can be applied to the substrate with only very great difficulty.
We have found that the novel polymer blends defined and claimed herein, that is, containing a low density ethylene polymer; a copolymer of butene-1 and ethylene or propylene; a hydrocarbon tackifier; and a low molecular weight polymer selected from the group consisting of a low molecular weight liquid polybutene, an amorphous polypropylene and mixtures thereof is eminently useful as a hot melt adhesive with the substrates defined above, in that they are compatible therewith, exhibit remarkable adhesion thereto and remain stable at elevated temperatures of application.